Polyethylene is the most widely used commercial polymer. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE). LDPE is a versatile polymer which can be used in a variety of applications, such as film, coating, molding, and wire and cable insulation. There is consequently still demand for optimizing the processes for its preparation.
A normal set-up of a plant for polymerizing or copolymerizing ethylenically unsaturated monomers such as ethylene in the presence of free-radical polymerization initiators consists essentially of a set of two compressors, a low-pressure and a high-pressure compressor, a polymerization reactor, which can be an autoclave or a tubular reactor or a combination of such reactors, and two separators for separating the monomer-polymer mixture leaving the tubular reactor, wherein in the first separator, the high-pressure separator, the ethylene separated from the monomer-polymer mixture is recycled to the ethylene-feed between the low-pressure compressor and the high-pressure compressor, and the ethylene separated from the mixture in the second separator, the low-pressure separator, is fed to the low-pressure compressor where it is compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the combined streams are further pressurized to the pressure of the high-pressure gas recycle stream. Such a high-pressure polymerization unit normally further includes apparatuses like extruders and granulators for pelletizing the obtained polymer. Monomer supply to the tubular reactor can either be carried out solely in the beginning of the reactor or only partly in the beginning with the other part fed via one or more side feed entries. Moreover, it is also common to introduce initiator in more than one place down the tube, thus creating more than one reaction zone.
The free-radical initiated polymerization of ethylene and optionally additional one or more comonomers is carried out at high pressures, which can reach even 500 MPa. Such high pressures require special technology for the process to be handled in a safe and reliable manner.
The compressors used for pressurizing ethylene and the other components of the reaction mixture are usually reciprocating piston compressors operating with plungers which are lubricated with oils. A recent development was to use as lubricant polyalkylene glycol (PAG) based oils instead of mineral oils. PAG based oils have the advantage that their solubility in supercritical ethylene is lower than the solubility of mineral oils. Therefore the lubrication films are more stable and less oil is transported to the polymer product and accordingly less oil is needed for the lubrication. More stable oil films also improve the lubrication and thereby increase the operating lifetime of the compressor. Moreover, at high pressures, PAG based oils have lower viscosities than mineral oils and can therefore more easily be pumped to the lubrication points.
The reliability of the high-pressure compressors is greatly reduced by polymer deposits in the compressors due to premature polymerization, i.e. polymerization prior to the feed of the free-radical polymerization initiators. In case of commonly used two-stage high-pressure compressors, these polymer deposits block filters on the suction side of the second compression stage as well as suction and discharge valves, resulting in increased pressures and temperatures between the two compression stages and causing strong vibrations on the cylinders of the second compression stage. However, such vibrations and increased interstage pressures can damage the compressor. Consequently, if such polymer deposits are formed it is necessary to remove them frequently. This requires however a shut-down of the plant accompanied by loss of production. Moreover, it had turned out that high-pressure compressors operated with PAG based oils are much more susceptible to formation of polymer deposits than high-pressure compressors operated with mineral oils.
As a consequence, there is a desire to avoid compressor fouling. One possibility is adding an inhibitor to the oil. Disadvantage is that the stabilizer in the oil will not be distributed homogeneously in the gas or supercritical phase thereby limiting its efficiency.
Another possibility is feeding inhibitor to the gas to be compressed. EP 811 590 A1 describes a process for compressing ethylenically unsaturated monomers which comprises carrying out compression in the presence of a sterically hindered amine derivative. EP 1 013 678 A2 discloses a similar process in which the pressurizing is effected in the presence of specific nitroxyl compounds. Additives such as nitrogen containing inhibitors can however remain in the polymer which is not desired as they can, for example, cause organoleptic issues. Furthermore, there is in general an increased demand in the market for “pure” LDPE.
WO 01/60875 refers to a method for polymerizing or copolymerizing ethylenically unsaturated monomers in the presence of radically decomposing polymerization initiators in a continuously operating polymerization device while preventing undesired polymer deposits in the compressors. The method is characterized in that nitrogen monoxide or oxygen is added in a dosed manner as an inhibitor into the high-pressure circuit, into the low-pressure circuit and/or into the pre-compressor. Oxygen can be used as inhibitor since it has generally an inhibiting effect at temperatures below 170° C. and an initiating effect only above 170° C. The document exemplifies oxygen concentrations of 3 ppm and 6 ppm and teaches that oxygen concentrations of from 1 to 5 ppm can be sufficient; the oxygen concentration is preferably 2 to 5 ppm. However, the disadvantage of using oxygen in such concentrations is that there is still an influence of the oxygen on the temperature profile in the tubular reactors. The more oxygen is fed the more rounded the first peak in the temperature profile gets and the less pronounced the temperature drop thereafter is. The success of peroxide initiated reactors is however at least partly due to the fact, that the steep temperature gradients allow for shorter reactors and therefore less investment costs. Oxygen initiated reactors show smaller temperature gradients with less sharp peaks. Therefore increasing the oxygen concentration partly cancels the advantages of peroxide initiation. Especially for high density grades which are run at low maximum temperature, a rounded temperature profile leads to too high temperatures at the beginning of the succeeding reaction zone, thereby limiting as well the range of obtainable product grades as also the achievable production rates.
Thus, it was the object of the present invention to overcome the disadvantages of such processes for polymerizing or copolymerizing ethylenically unsaturated monomers in the presence of free-radical polymerization initiators and provide a possibility for avoiding premature polymerization in the compressors and thereby allowing a higher throughput of the compressors and reducing the risk of compressor damages by reducing compressor vibrations while simultaneously detrimental effects on the flexibility with respect to the range of obtainable product grades and on achievable production rates are minimized